Specifications

F3
GE Limitamp
®
Medium Voltage Motor Control
Protection & Control
F
hermetically sealed air-conditioning motors, and is well suited
as a stall-protection relay.
Multifunction Solid-State Relays
Large motors on vital drives need accurate protection
against overloads, phase unbalance or ground faults.
Multifunction solid-state relays are available from GE that
offer total motor protection in one compact package. Basic
protective functions such as overtemperature, overload,
instantaneous overcurrent, open-phase, phase reversal,
phase unbalance, ground-fault, load jam, load loss and
bearing overtemperature protection can be provided.
Overtemperature Relays
Some motors have RTDs placed in the stator slots. The purpose
is to obtain an indication of winding temperature by meas-
uring the RTD resistance and its change with temperature.
Difficulty arises in obtaining a continuously accurate indication
of temperatures, however, because of the time lag of heat
transfer from the stator conductors to the RTD caused by the
insulating material surrounding the conductors. Temperature
changes in the conductor will not be reflected in RTD resist-
ance change until heat is transferred through the thermal
resistance and capacitance of the insulating material.
Figure F.3 Approximate temperature of RTD in large motor during locked rotor
If the copper temperature is changing very rapidly, such as
during locked rotor, the RTD will lag far behind the copper
temperature as shown in Figure F.4. Consequently, monitoring
the RTD temperature is inadequate for thermal protection
during rapid-transient conditions. However, for steady-state
indication of temperature, the RTD is very accurate.
A relay which responds to changes in resistance of RTDs,
providing steady-state indication of motor-winding temper-
ature, used in conjunction with a bimetallic overload relay
will provide reasonably precise over-temperature protection
for the motor.
Available solid-state relays contain a device which will more
accurately compute hot-spot temperature by utilizing RTD
amperes and line amperes. This relay accurately tracks
motor heating and is recommended in preference to the
separate bimetal relay and RTD relay.
Open-Phase And Phase-Unbalance Prot
ection
A three-phase motor may be damaged when subjected to
unbalanced line currents. Usually, the damage occurs in the
rotor from overheating, caused by reverse sequence compo-
nents of currents not detected by normal overload devices.
The rate of motor heating will be a function of the degree of
phase unbalance, the most extreme of which is the open-
phase condition. For that reason, open-phase relays should
operate instantaneously to avoid serious motor damage.
Likewise, a motor may be damaged over a period of time with
as little as 10% unbalance, where unbalance is a transient
condition which would not justify instantaneous shutdown.
Consequently, the time to trip should be delayed in proportion
to the percentage of unbalance.
More comprehensive open-phase or single-phase protection
can be obtained by applying a solid-state motor-protective
relay, which will trip the contactor in the event of an open
phase, regardless of the cause, even if external to the vacuum
Limitamp control.
A possible concern that may arise when applying a medium-
volt contactor to a transformer feeder is what happens to
the contactor when a voltage dip occurs. In the past, the
contactor would drop out — removing power from the primary
of the transformer when the contactor coil power is reduced
to 60 to 80 percent of full voltage. To prevent dropout during
loss of control voltage, latching contactors should be applied.
In these cases, the contactor is latched by a closing coil and
unlatched by a trip coil. A capacitor trip device can be applied
to trip the contactor in the event of total loss of control
power. (See Latched Contactors, page B4Update reference
to current page.)
Current Differential Protection
The term differential, as applied to a type of protective
relaying, designates the principle on which the scheme
operates — that is, a differ
ence in current. The relays used
are connected in such a way as to detect a percentage
differential in current between ends of a motor winding.
Ordinarily, in a machine operating without a winding fault,
the current into one end of a phase winding is equal to the
current out the other end of the same winding. When a fault
occurs, however, the current into one end of the winding is
short circuited inside the machine (to another phase or to
ground) at the place of fault, so that a differential occurs
between current “in” and current “out.” This causes the relay
to operate. The percentage differential may at times be
quite small when the fault is located at a point of high
impedance inside the motor winding, and this is the reason
why straight over-current relays alone do not always give
adequate protection.
0 20 40 60 80 100 120 140
160
120
80
40
0
Power Off
Time in Seconds
Temperature
in Degrees C
Tooth
RTD
Hot Spot of WDG in Slot
Power
On